Ethylene production continues to grow and has replaced acetylene for many applications. Ethylene production occurs mainly through pyrolysis which is the thermal cracking of various hydrocarbon streams in the presence of steam. The heat transfer in the radiant section of a thermal cracking furnace is critical. Cracking furnaces present both burner design and operating challenges in comparison to typical process heaters. The thermal cracking of hydrocarbons in the radiant section process tubes of a cracking furnace occurs at a higher temperature compared to most other refinery or chemical processes. In order to crack hydrocarbons in the presence of steam, the temperature of the combustion products in the radiant section of the furnace must be high to achieve the required heat transfer.
Fuel gas burned at high temperatures in an excess air environment results in the production of Nitrogen Oxides (NOx). NOx is considered hazardous to the environment and, thus, environmental regulations have been placed on the quantity of NOx that could be produced in the combustion process in fired heaters and furnaces. Due to various regulations, burner designs used in cracking furnaces have evolved in recent years, improving the efficiency of combustion while reducing the amount of NOx produced. In one approach, staged combustion has been used to reduce the amount of NOx formed in the combustion process by reducing the flame temperature and reducing the concentration of oxygen available. Staged combustion involves delaying the mixing of the fuel and air and promotes the mixing of combustion products with the fuel and air mixture to provide a reduction in flame temperature and a reduction in the partial pressure of oxygen. Combustion products are the products of combustion from the burner which fill the inside of the furnace prior to discharge at the top of the furnace. Combustion products may be comprised of components such as carbon dioxide, water vapor, nitrogen and oxygen.
Historically, thermal cracking furnaces were fired with a large number of premix radiant wall burners. Premix radiant wall burners are well known for their short, compact flame, which can produce uniform heat flux throughout the radiant section of the furnace. Although premix burners are a common design in cracking furnaces there are significant cost issues associated with the use of premix burners because a large number of burners must be installed.
Current low NOx burner designs employed in cracking furnaces are typically nozzle mix “deeply” staged fuel configurations. Low NOx cracking furnace burners discharge fuel from two distinct locations. Typically one discharge location is in the burner tile throat area. This location discharges an initial source of fuel, called primary fuel, which comprises 10%-20% of the total fuel burned. The burners typically include one or more primary fuel nozzle mix tips that are located in a burner air passage that pass through the throat of a burner tile. This primary fuel burns in an environment with high excess air, which could lead to increased NOx formation if the fuel and air are not completely mixed. The remainder of the fuel needed for the process is injected at a secondary location which is external to the burner tile and downstream from an air passage discharge used to discharge the primary fuel. The fuel discharged at the second location is called secondary or staged fuel. Secondary fuel is normally discharged through multiple nozzle fuel tips that are located external of the burner tile. Such burner assemblies are normally referred to as “deeply” staged because they use two locations for the discharge of fuel and the majority of the fuel they utilize is staged at the secondary or staged location. For minimal NOx emissions, “deeply” staged fuel burners mix combustion products with the staged fuel prior to combustion in the secondary combustion zone. In such a design, the staged tip configurations that are necessary to minimize flame length and stabilize the flame in the secondary combustion zone entrain an insufficient amount of combustion products that are mixed with the staged fuel. Subsequently, the burner does not achieve maximum reduction in NOx emissions. These burners are either floor fired burners (hearth burners) or floor fired burners in combination with side wall or balcony burners. These burners employ a rectangular discharge opening of the burner tile that sits against the furnace wall and provides a flat flame. The low NOx premix assembly of the present invention incorporates staged fuel combustion and combustion product recirculation to reduce the level of NOx generated, while providing minimum flame length and maximum stability.
Recently, there has been an effort to reduce the physical size of the thermal cracking furnace which consequently reduces the furnace volume while increasing the heat density. Subsequently, by decreasing the length of the furnace, the space between burners has been reduced causing flame overlap and interference. This flame overlapping tends to cause NOx emissions to increase. Further, another effect of flame overlap is for the flame length to increase, so much so that the flames between the burners tend to protrude further into the furnace space between the furnace wall and the process tubes. Combustion product flow patterns in the radiant section of the furnace have a significant impact on the burner flame pattern. Combustion products flow upwards along a hot firing wall, while a downward flow recirculates back toward the furnace floor along the surface of the lower temperature tubes. If the burner flames become too long then the combustion product flow within the furnace is able to draw the flames across the furnace to the tubes causing overheating of the tubes which may lead to tube failure.
Additionally, prior art burner designs have further complications. The nozzle mix “deeply” staged fuel burner configuration results in a low discharge velocity as the primary fuel combustion products and any excess combustion air exits the burner tile. Also the prior burner design results in a delayed mixing of combustion air with the deeply staged fuel. Therefore, with the combination of circulation patterns in the furnace, low discharge velocity, and the delayed mixing of combustion air and staged fuel, a complication called “flame rollover” commonly results. Flame rollover can occur in the upper portions of the flame resulting in flame impingement or hot gas impingement on the process tubes.
Yet another complication of the “deeply” staged fuel configurations is that the delayed burning of the staged fuel creates a relatively low combustion temperature above the top of the burner tile and therefore the desired radiant flux profile may not be available for appropriate heat transfer giving a lower than desired efficiency.
Accordingly, it is therefore desirable to provide a cracking furnace burner assembly with a burner tile design that allows for an efficient mechanism to mix combustion products with the air and fuel within the burner tile prior to combustion in the furnace thereby providing an extremely uniform high velocity mixture that reduces the flame length as well as subsequent complications such as flame rollover.
It is further desirable to provide a cracking furnace burner assembly that uses premix methods for discharging either or both primary and staged fuel providing a uniform fuel, air and combustion product-mixture prior to combustion thereby minimizing NOx emissions and flame length.
It is yet further desirable to provide a burner assembly design that uses premix fuel burner tips that allow gas mixtures to exit the burner tile at an extremely high velocity to prevent flame rollover.
It is yet further desirable to provide a burner assembly design that utilizes combustion products from within the furnace in order to cool the system, thereby minimizing NOx levels.
It is yet further desirable to provide a burner assembly design that allows for the complete mixing of the primary fuel and the air promoting the initial 50% of combustion to occur close to the tile discharge of the burner tile and under sub-stoichiometric conditions.
It is yet further desirable to provide a burner assembly design that eliminates delayed combustion accounted for in deeply staged fuel designs.